FIG. 1 is a cross section schematic view of a thin film transistor. As shown in FIG. 1, the thin film transistor includes: a gate electrode 2 disposed on a base substrate 1, a gate insulating layer 3 disposed on the gate electrode 2, an active layer 4 disposed on the gate insulating layer 3, a source electrode 5 and a drain electrode 6 disposed on the active layer 4. In the case that the voltage applied to the gate electrode is higher than the threshold voltage of the thin film transistor, the source electrode 5 and the drain electrode 6 are electrically connected with each other through the active layer 4 and the carriers flow from the source electrode 5 to the drain electrode 6 or flow from the drain electrode 6 to the source electrode 5.